Mixing devices such as mixers, extruders, gear pumps, and devolatizers have a long history of use in processing various types of polymeric materials. Typically the polymeric material is melted within the mixing device or prior to entry into the mixing device to facilitate processing of the polymeric material in the mixing device. Polymeric materials may be melted by simply heating the material to its melting point. Additives, including reactants and degradents, may also be introduced into the heated polymeric material.
During operation, the mixing device typically exerts pressure and stresses onto the polymeric material as necessary for product formulation. Passing the polymer through dies or into molds enables the molten polymer to be formed into a desired final shape and size of the polymer product. It is important to monitor rheological properties of the molten polymer such as melt index, density, viscosity, elasticity, polymer composition, and polymer additive composition because final product performance is dependent on these properties and because properly forming the desired shape and size of a polymer product is dependent upon how well the molten polymer flows through a die or into a mold. On-line monitoring of these properties can enable the operator to adjust operational parameters thereby ensuring that these properties are within specification.
One method currently used to measure some of these physical properties involves directing a portion of the polymer, in molten or solid form, to a process rheometer. However rheometer maintenance requirements are high, which not only increases operating costs, but due to maintenance down time exacerbates its unreliable test data results. Typically, rheometers measure the properties of the molten polymer by analyzing a slip stream are taken from a mixing device, particularly extruder mixing devices.
EP 0 238 796 A2; EP 0 406 805 A2; JP 02 038 841; and U.S. Pat. No. 6,405,579 disclose the use of rheometers to determine rheological properties of polymeric materials.